2. Field of the Invention
The present invention relates generally to capacitive measuring systems and, more specifically, to systems having a sensor shielded against the detrimental effects of external electric fields and circuits for capacitively measuring the volume charge density of a sample of material.
2. Description of the Related Art
Capacitive measuring systems have been used to measure dissolved solids and impurities in fluids such as water and oil. A capacitive sensor is a device having two electrically conductive electrodes and a non-conductive body that insulates the fluid from the electrodes. The electrodes are typically tubular in shape and concentric with one another. When a sample is placed in the body, the device defines a capacitance in response to the dielectric constant of the sample. The dielectric constant varies in response to the ion concentration in the sample, which, in turn, is related to the solid impurities. The capacitance may be measured by connecting a suitable oscillator and measuring circuit to the plates. Comparing the measured capacitance to a known capacitance provides information relating to the electrical properties of the sample. For example, the dissolved solids in a sample of water can be determined by comparing the measured value to that which is produced in response to a known pure (e.g., double-distilled) sample of water.
Conventional capacitive sensors of the type described above are of low precision. They cannot, for example, consistently measure ion concentrations in water below a few parts per million. Practitioners in the art have discovered that measurements may vary over a wide range under seemingly identical test conditions. It would be desirable to provide a capacitive measuring system having a high-precision sensor. This and other problems and deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.
The present invention includes a capacitive sensor and an electronic measurement system. The sensor includes a tubular outer conductor, a tubular inner conductor coaxial with the outer conductor, and an electrically insulated chamber between the inner and outer conductors. The material sample to be measured is placed in the chamber or forced to flow through the chamber. The chamber electrically isolates the sample from the conductors. When the sample is introduced into the chamber, it defines the dielectric of a capacitor. The plates of the capacitor are defined by the inner and outer conductors.
It has been discovered in accordance with the present invention that measurements produced by capacitive sensors known in the art are detrimentally affected by external electric fields, i.e., fields produced by environmental sources external to the sensor, such as fluorescent lights. A conductor exposed to an electric field acts as an antenna and develops a potential. If measurements are taken using such a sensor in, for example, a room having fluorescent lighting, the measurements will be markedly different than if taken in a room not having fluorescent lighting. Even in the absence of fluorescent lighting and other apparent sources of electric fields, the body of the person taking the measurements may emit sufficient electromagnetic radiation to affect the measurements.
In accordance with the discovery that electric fields detrimentally affect capacitive impurity measuring systems, in certain embodiments, the present invention includes electrostatic shielding that completely encloses the chamber in which the sample is contained during measurement. In an exemplary embodiment, the sensor has one or more openings, and a removable cap made of a conductive material is attachable the opening. No external electric field can penetrate into the chamber because the cap is in electrical contact with the outer conductor and seals the opening during measurement. In another exemplary embodiment, the sensor has one or more openings, and a valve selectably opens or closes the opening. No external electric field can penetrate into the chamber because the valve, which may be solenoid-operated, has a conductive member that is in electrical contact with the outer conductor and seals the opening during measurement. In yet another exemplary embodiment, the sensor has one or more openings in which a screen made of a conductive material is disposed. An external electric field cannot penetrate into the chamber to any significant extent because the screen is in electrical contact with the outer conductor.
The electronic measurement system determines the difference between a reference signal or a value representing such a signal having a constant reference frequency and a test signal or value representing such a signal having a test frequency responsive to the sensor capacitance. In an exemplary embodiment, the circuit includes D-type flip-flops that, in effect, subtract one signal from the other. In another exemplary embodiment, the circuit includes a phase-locked loop. The circuit may also include analog or digital means for improving response linearity, such as a logarithmic amplifier, look-up table, or polynomial correction. Also, the electronic measurement system may, in certain embodiments, periodically reverse the polarity of the test signal applied to the sensor in order to minimize charging of non-conductive surfaces, which promotes measurement accuracy and provides a cleaning effect and other benefits.
The system may be used for a variety of purposes, including measuring the extent of impurities in fluids, such as gases and water and other liquids, and measuring the flow rate of such fluids.